Sforno on Genesis 17:5/ספורנו על בראשית י״ז:ה׳

1והיה שמך אברהם, "henceforth you name will be Avraham. Immediately."

2 כי אב המון גויים נתתיך, for the significance of this name change will commence as of this day. The word נתתיך in the past tense means that this process has already commenced. This is why לא יקרא עוד שמך אברם, your name from now on will now longer be Avram, ever. This was a different name change from when Yaakov was accorded the name ישראל. That name was an additional name which did not replace his original name Yaakov. The matter will be explained in due course.